I pay a few dollars a year for Eset, which I am quite happy with. I have tried a number of the "free" or "freely available" antivirus programs, including Avast, Symantec, and others. I even paid for an on sale (very cheap) copy of Bitdefender last year. I always come back to Eset, but if had to go with something at no cost, I would definitely choose Symantec over Avast.
Ive always found Avast to be great for me, and on a number of systems Ive used and built. Cheers Scorpion_Blue